1. Field of the Invention
This invention relates to a charging member and device for imparting electrical charge to an object, typically a photoconductor for use in electrophotography and electrostatic recording process, for example, copying machines and printers.
2. Prior Art
Conventional electrophotography as applied to copying machines and printers involves the steps of uniformly charging a photoconductor on the surface, projecting an image from an optical system onto the photoconductor to form a latent image in an exposed area where the electric charge is erased, applying toner to the photoconductor to form a toner image, and transferring the toner image to a record medium, typically paper.
The first step of electrically charging the photoconductor typically employs a corona discharge system. The corona discharge system, however, is undesirable from the standpoint of safety and maintenance of the machine since it requires application of as high voltage such as 5 to 10 kV. It also suffers from an environmental problem due to the emission of harmful substances such as ozone and NOx during corona discharge.
There is a need for an alternate charging system capable of charging at a lower applied voltage than for the corona discharge and minimizing emission of harmful substances such as ozone and NOx. One exemplary alternate charging system is a contact charging system as disclosed in Japanese Patent Application Kokai (JP-A) Nos. 205180/1989 and 211779/1989 wherein a charging member having voltage applied thereto is brought in contact with an object to be charged, such as a photoconductor, thereby charging the object. Known charging members for use in the contact charging system include conductive rubber rollers having conductive particles such as carbon dispersed in rubber and rollers having a layer of nylon or polyurethane coated on the conductive rubber rollers.
However, problems also arise with the contact charging system. For example, a conductive rubber roller having a coating layer performs well initially when used as a charging member. As the printing operation is repeated, the toner carried by the photoconductor drum past the cleaning blade will deposit and fuse to the surface of the conductive rubber roller. If the roller is used without removing such toner deposits, fog and other defects occur in the printed image. There is a likelihood that the toner deposits will be separated from the conductive roller and fused to the photoconductor, also causing defects to the printed image.
In this regard, it is empirically known that image defects can be eliminated to some extent by lowering the hardness of the entire roller, typically the Ascar C hardness of the conductive rubber roller.
The technique of lowering the Ascar C hardness of the roller for improvement in image quality is merely an empirical practice of those skilled in the art, but not a well established technique. In other words, lowering Ascar C hardness does not always improve image quality and can sometimes worsen image quality.
In order to provide a roller with a low Ascar C hardness, the hardness of a coating layer must be considerably low. A conductive rubber roller having a coating layer with such low hardness can closely adhere to the photoconductor at elevated temperature. Another means for lowering Ascar C hardness is to reduce the thickness of a coating layer, which can permit the roller to be broken upon voltage application. It is also possible to lower Ascar C hardness by lowering the hardness of a rubber elastomer layer. To this end, an amount of an extra component such as oil must be added to a rubber composition of which the elastomer layer is formed, and in a certain operational situation, the resulting conductive rubber roller allows the extra component to bleed out, staining the photoconductor. In order to provide a low hardness, it is essential to use flexible material, which in turn, recommends the use of a plasticizer. The plasticizer if added, however, this raises a staining problem in that it will migrate to the photoconductor and also causes the inconvenience that due to its tackiness, the charging member will tightly adhere to the photoconductor during long-term storage.
It is then difficult to provide an improved charging member by relying on the technique of lowering the hardness, typically the Ascar C hardness of the entire roller. The evaluation of a charging member in terms of Ascar C hardness has therefore not been well established.
Meanwhile, insofar as the inventors' research works are concerned, a conductive urethane foam which is rendered conductive by adding a quaternary ammonium salt, an electron acceptor capable of forming a charge transfer complex, an inorganic salt and/or carbon black is optimum among the charging members which satisfy all the requirements of a desired electric resistance, low hardness and low compression set.
A charging roller formed of such semi-conductive urethane foam, however, is less likely to stain the photoconductor. To overcome this problem, the charging roller is conventionally washed with a suitable solvent such as acetone prior to use. A large amount of used solvent and a cost of washing are problems.